Integral composite membrane

ABSTRACT

A composite membrane is provided which includes a base material and an ion exchange resin. The base material has a microstructure characterized by nodes interconnected by fibrils, or a microstructure characterized by fibrils with no nodes present. The ion exchange resin substantially impregnates the membrane such that the membrane is essentially air impermeable.

This is a continuation of Reissue U.S. Ser. No. 09/245,496, filed Feb.4, 1999, now abandoned, which was a Reissue application based on U.S.Pat. No. 5,599,614 (issued on Feb. 4, 1997) which was U.S. Ser. No.08/561,514, filed Nov. 21, 1995, which This is a continuation in part ofapplication Ser. No. 08/404,853, filed on Mar. 15, 1995, now U.S. Pat.No. 5,547,551; said U.S. Ser. No. 08/561,514 is also acontinuation-in-part of U.S. Ser. No. 08/339,425, filed Nov. 14, 1994,now abandoned.

OTHER RELATED APPLICATIONS

Above mentioned U.S. Ser. No. 08/404,853, filed on Mar. 15, 1995, nowU.S. Pat. No. 5,547,551, is currently being subject to Reissue in U.S.Ser. No. 09/137,515, filed on Aug. 20, 1998, which will issue as RE37307on Aug. 7, 2001. Above mentioned U.S. Ser. No. 08/339,425, filed Nov.14, 1994, which was abandoned in favor of continuation application Ser.No. 08/903,844, filed Jul. 31, 1997, which was abandoned in favor ofcontinuation application Ser. No. 09/209,932, filed Jul. 8, 1998, nowU.S. Pat. No. 6,254,978.

FIELD OF THE INVENTION

An integral composite membrane is provided which is useful inelectrolytic processes and other chemical separations.

BACKGROUND OF THE INVENTION

Ion exchange membranes (IEM) are used in polymer electrolyte fuel cellsas solid electrolytes. A membrane, located between a cathode and ananode of such a fuel cell, transports protons formed near the catalystat the hydrogen electrode to the oxygen electrode, thereby allowing acurrent to be drawn from the fuel cell. These polymer electrolyte fuelcells are particularly advantageous because they operate at lowertemperatures than other fuel cells. Also, these polymer electrolyte fuelcells do not contain any corrosive acids which are found in phosphoricacid fuel cells. In these type fuel cells, there is a need to eliminatethe bulk transfer of reactants from one electrode to the other, i.e.fluid percolation.

Ion exchange membranes are also used in chloralkali applications toseparate brine mixtures to form chlorine gas and sodium hydroxide. Forbest performance, it is preferred that the membrane selectivelytransport the sodium ions across the membrane while rejecting thechloride ions. Also, the ion exchange membrane must eliminate bulktransfer of electrolytic solution across the membrane, i.e. fluidpercolation.

Additionally, IEMs are useful in the areas of diffusion dialysis,electrodialysis and in pervaporation and vapor permeation separations.IEMs may also be used for selective transport of polar compounds frommixtures containing both polar and non-polar compounds.

IEMs must have sufficient strength to be useful in their variousapplications. Often, this need for increased strength requires that anIEM be made relatively thick in cross section, or that the IEM bereinforced with woven fabrics (macro-reinforcements), both of whichdecreases the ionic conductance of the IEM. Additionally, conventionalIEMs exhibit inherent dimensional instability due to the absorbance ofsolvents, such as water, for example. Such dimensional instabilityrenders conventional IEMs substantially ineffective for many commercialapplications.

U.S. Pat. No. 3,692,569 relates to the use of a coating of a copolymerof fluorinated ethylene and a sulfonyl-containing fluorinated vinylmonomer on a fluorocarbon polymer that was previously non-wettable. Thefluorocarbon polymer may include tetrafluoroethylene polymers. Thiscoating provides a topical treatment to the surface so as to decreasethe surface tension of the fluorocarbon polymer. U.S. Pat. No. 3,692,569provides for a fluid percolating structure.

U.S. Pat. No. 4,453,991 relates to a process for making articles coatedwith a liquid composition of a perfluorinated polymer, having sulfonicacid or sulfonate groups in a liquid medium, by contacting the polymerwith a mixture of 25 to 100% by weight of water and 0 to 75% by weightof a second liquid component, such as a low molecular weight alcohol, ina closed system. Such a process provides for a multi-layered structure.

U.S. Pat. No. 4,902,308 relates to a film of porous expandedpolytetrafluoroethylene (PTFE) having its surfaces, both exterior andinternal, coated with a metal salt of perfluoro-cation exchange polymer.Such a composite product is permeable to air. The air flow of such astructure, as measured by the Gurley densometer ASTM D726-58, is about12 to 22 seconds. Therefore, this structure provides for fluidpercolation.

U.S. Pat. No. 5,082,472 relates to a composite material of a microporousmembrane, such as porous expanded PTFE, in laminar contact with acontinuous ion exchange resin layer, wherein both layers have similararea dimensions. Surfaces of internal nodes and fibrils of the expandedPTFE may be coated, at least in part, with an ion exchange resincoating. The expanded PTFE layer of this composite membrane impartsmechanical strength to the composite structure. However, the interior ofthe expanded PTFE membrane is unfilled so as to not block the flow offluids. Therefore, U.S. Pat. No. 5,082,472 provides for fluidpercolation.

U.S. Pat. No. 5,094,895 and 5,183,545 relate to a composite porousliquid-permeable article having multiple layers of porous expanded PTFE,which are bonded together, and which have interior and exterior surfacescoated with an ion exchange polymer. Such a composite article isparticularly useful as a diaphragm in electrolytic cells. However,diaphragms are inherently percolating structures.

Japanese Patent Application No. 62-240627 relates to a coated or animpregnated membrane formed with a perfluoro type ion exchange resin anda porous PTFE film to form an integral structure. The resultingcomposite is not fully occlusive. Furthermore, the teachings of thisapplication do not provide for permanent adhesion of the ion exchangeresin to the inside surface of the PTFE film.

There remains a need for a strong, integral composite ion exchangemembrane, having long term chemical and mechanical stability.

SUMMARY OF THE INVENTION

The present invention is an advancement over presently known ionexchange membranes. In one embodiment of the present invention, this isaccomplished by providing a composite membrane comprising an expandedpolytetrafluoroethylene (PTFE) membrane having a porous microstructureof polymeric fibrils. The composite membrane is impregnated with an ionexchange material throughout the membrane. The impregnated expandedpolytetrafluoroethylene membrane has a Gurley number of greater than10,000 seconds. The ion exchange material substantially impregnates themembrane so as to render an interior volume of the membranesubstantially occlusive.

The expanded PTFE membrane may comprise a microstructure of nodesinterconnected by fibrils.

The ion exchange material may be selected from a group consisting ofperfluorinated sulfonic acid resin, perfluorinated carboxylic acidresin, polyvinyl alcohol, divinyl benzene, styrene-based polymers, andmetal salts with or without a polymer. The ion exchange material mayalso be comprised of at least in part a powder, such as but not limitedto, carbon black, graphite, nickel silica, titanium dioxide, andplatinum black.

A purpose of the present invention is to provide an improved alternativeto the macro-reinforcement of ionomer materials.

Another purpose of the present invention is to provide an ion exchangemembrane having a single integral structure that does not allow forfluid percolation.

The foregoing and other aspects will become apparent from the followingdetailed description of the invention when considered in conjunctionwith the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-section of a composite membrane of thepresent invention that is fully impregnated with an ion exchangematerial.

FIG. 2 is a schematic cross-section of the composite membrane of thepresent invention that is fully impregnated with an ion exchangematerial and which includes a backing material attached thereto.

FIG. 3 is a photomicrograph, at a magnification of 2.5 kX, of across-section of an expanded PTFE membrane that has not been treatedwith an ion exchange material.

FIG. 4 is a photomicrograph, at a magnification of 5.1 kX, of across-section of an expanded PTFE membrane impregnated with an ionexchange material, such that the interior volume of the membrane issubstantially occluded.

FIG. 5 is a photomicrograph, at a magnification of 20.0 kx, of across-section of an expanded PTFE membrane, comprised substantially offibrils with no nodes present, which has not been treated with an ionexchange material.

DETAILED DESCRIPTION OF THE INVENTION

As best illustrated by FIG. 1, a composite membrane is provided whichincludes a base material 4 and an ion exchange material or ion exchangeresin 2. The base material 4 is a membrane which is defined by a porousmicrostructure characterized by nodes interconnected by fibrils (FIG.3), or a porous microstructure characterized substantially by fibrils(FIG. 5). The ion exchange resin substantially impregnates the membraneso as to render the interior volume substantially occlusive. The ionexchange resin is securely adhered to both the external and internalmembrane surfaces, i.e. the fibrils and/or nodes of the base material.

The composite membrane of the present invention may be employed invarious applications, including but not limited to, polarity-basedchemical separations; electrolysis; fuel cells and batteries;pervaporation; gas separation; dialysis separation; industrialelectrochemistry, such as chloralkali production and otherelectrochemical applications; use as a super acid catalyst; or use as amedium in enzyme immobilization, for example.

The composite membrane of the present invention is uniform andmechanically strong. As used herein, the term “uniform” is defined ascontinuous impregnation with the ion exchange material such that no pinholes or other discontinuities exist within the composite structure. Themembrane should be “occlusive”, meaning that the interior volume of theporous membrane is impregnated such that the interior volume is filledwith the ion exchange material and the final membrane is essentially airimpermeable having a Gurley number of greater than 10,000 seconds. Afill of 90% or more of the interior volume of the membrane shouldprovide adequate occlusion for purposes of the present invention.

A preferred base material 4 is an expanded polytetrafluoroethylene(ePTFE) which may be made in accordance with the teachings of U.S. Pat.No. 3,593,566, incorporated herein by reference. Such a base materialhas a porosity of greater than 35%. Preferably, the porosity is between70-95%. Preferably the thickness is between 0.06 mils (0.19 μm) and 0.8mils (0.02 mm), and most preferably the thickness is between 0.50 mils(0.013 mm) and 0.75 mils (0.019 mm). This material is commerciallyavailable in a variety of forms from W. L. Gore & Associates, Inc., ofElkton, Md., under the trademark GORE-TEX®. FIG. 3 shows aphotomicrograph of the internal porous microstructure of an embodimentof such an expanded PTFE membrane. As seen therein, the porousmicrostructure comprises nodes interconnected by fibrils which define aninterior volume of the base material 4. Alternatively, the base material4 may comprise an ePTFE material having a porous microstructure definedsubstantially of fibrils with no nodes present.

To manufacture an ePTFE membrane having a porous microstructure definedsubstantially of fibrils with no nodes present, a PTFE that has a lowamorphous content and a degree of crystallization of at least 98% isused as the raw material. More particularly, a coagulated dispersion orfine powder PTFE may be employed, such as but not limited to FLUON®CD-123 AND FLUON® CD-1 available from ICI Americas, Inc., or TEFLON®fine powders available from E. I. DuPont de Nemours and Co., Inc.(TEFLON is a registered trademark of E. I. DuPont de Nemours and Co.,Inc.) These coagulated dispersion powders are lubricated with ahydrocarbon extrusion aid, preferably an odorless mineral spirit, suchas ISOPAR K (made by Exxon Corp.) (ISOPAR is a registered trademark ofthe Exxon Corporation). The lubricated powder is compressed intocylinders and extruded in a ram extruder to form a tape. The tape iscompressed between rolls to an appropriate thickness, usually 5 to 10mils. The wet tape is stretched traversely to 1.5 to 5 times itsoriginal width. The extrusion aid is driven off with heat. The driedtape is then expanded longitudinally between banks of rolls in a spaceheated to a temperature that is below the polymer melting point(approximately 327° C.). The longitudinal expansion is such that theratio of speed of the second bank of rolls to the first bank is fromabout 10-100 to 1. The longitudinal expansion is repeated at about 1-1.5to 1 ratio. After the longitudinal expansion, the tape is expandedtraversely, at a temperature that is less than about 327° C., to atleast 1.5 times, and preferably to 6 to 15 times, the width of theoriginal extrudate, while restraining the membrane from longitudinalcontraction. While still under constraint, the membrane is preferablyheated to above the polymer melting point (approximately 342° C.) andthen cooled. This ePTFE membrane is characterized by the followingproperties:

(a) average pore size between 0.05 and 0.4 micrometers, and preferablyless than 0.2;

(b) a bubble point between 10 and 60 psi;

(c) a pore size distribution value between 1.05 and 1.20;

(d) a ball burst strength between 0.9 and 17 pounds/force;

(e) an air flow of between 20 Frazier and 10 Gurley seconds;

(f) a thickness between 1.32 μm and 25.4 μm; and

(g) a fiber diameter of between 5 and 20 Nm.

Suitable ion exchange materials 2 include, but are not limited to,perfluorinated sulfonic acid resin, perfluorinated carboxylic acidresin, polyvinyl alcohol, divinyl benzene, styrene-based polymers andmetal salts with or without a polymer. A mixture of these ion exchangematerials may also be employed in treating the membrane 4. Solvents thatare suitable for use with the ion exchange material, include forexample, alcohols, carbonates, THF (tetrahydrofuran), water, andcombinations thereof. Optionally, ion exchange materials may becomplemented by finely divided powders or other (non-ionic) polymers toprovide final composites. Such a finely divided powder may be selectedfrom a wide range of organic and inorganic compounds such as, but notlimited to, carbon black, graphite, nickel, silica, titanium dioxide,platinum black, for example, to provide specific added effects such asdifferent aesthetic appearance (color), electrical conductivity, thermalconductivity, catalytic effects, or enhanced or reduced reactanttransport properties. Examples of non-ionic polymers include, but arenot limited to, polyolefins, other fluoropolymers such as polyvinylidene(PVDF), or other thermoplastics and thermoset resins. Such non-ionicpolymers may be added to aid occlusion of the substrate matrix, or toenhance or reduce reactant transport properties.

A surfactant having a molecular weight of greater than 100 is preferablyemployed with the ion exchange material 2 to ensure impregnation of theinterior volume of the base material 4. Surfactants or surface activeagents having a hydrophobic portion and a hydrophilic portion may beutilized.

A most preferred surfactant is a nonionic material, octylphenoxypolyethoxyethanol having a chemical structure:

where x=10 (average),

and is known as Triton X-100, which is commercially available from Rohm& Haas of Philadelphia, Pa.

As best seen by reference to FIG. 4, the final composite membrane of thepresent invention has a uniform thickness free of any discontinuities orpinholes on the surface. The interior volume of the membrane is occludedsuch that the composite membrane is impermeable to non-polar gases andto bulk flow of liquids.

Optionally, and as shown schematically in FIG. 2, the composite membranemay be reinforced with a woven or non-woven material 6 bonded to oneside of the base material 4. Suitable woven materials may include, forexample, scrims made of woven fibers of expanded porouspolytetrafluoroethylene; webs made of extruded or oriented polypropyleneor polypropylene netting, commercially available from Conwed, Inc. ofMinneapolis, Minn; and woven materials of polypropylene and polyester,from Tetko Inc., of Briarcliff Manor, N.Y. Suitable non-woven materialsmay include, for example, a spun-bonded polypropylene from Reemay Inc.of Old Hickory, Tenn.

The treated membrane may be further processed to remove any surfactantwhich may have been employed in processing the base material asdescribed in detail herein. This is accomplished by soaking orsubmerging the membrane in a solution of, for example, water, isopropylalcohol, hydrogen peroxide, methanol, and/or glycerin. During this step,the surfactant, which was originally mixed in solution with the ionexchange material, is removed. This soaking or submerging causes aslight swelling of the membrane, however the ion exchange materialremains within the interior volume of the base material 4.

The membrane is further treated by boiling in a suitable swelling agent,preferably water, causing the membrane to slightly swell in the x and ydirection. Additions swelling occurs in the z-direction. A compositemembrane results having a higher ion transport rate that is also strong.The swollen membrane retains its mechanical integrity and dimensionalstability, unlike the membranes consisting only of the ion exchangematerial. Also, the membrane maintains desired ionic transportcharacteristics. A correlation exists between the content of theswelling agent within the membrane structure and transport properties ofthe membrane. A swollen membrane will transport chemical species fasterthan an unswollen membrane.

Although the membrane has excellent long term chemical stability, it canbe susceptible to poisoning by organics. Accordingly, it is oftendesirable to remove such organics from the membrane. For example,organics can be removed by regeneration in which the membrane is boiledin a strong acid, such as nitric or chromic acid.

To prepare the integral composite membrane of the present invention, asupport structure, such as a polypropylene woven fabric, may first belaminated to the untreated base material 4 by any conventionaltechnique, such as, hot roll lamination, ultrasonic lamination, adhesivelamination, or forced hot air lamination so long as the technique doesnot damage the integrity of the base material. A solution is preparedcontaining an ion exchange material in solvent mixed with one or moresurfactants. The solution may be applied to the base material 4 by anyconventional coating technique including forwarding roll coating,reverse roll coating, gravure coating, doctor coating, kiss coating, aswell as dipping, brushing, painting, and spraying so long as the liquidsolution is able to penetrate the interstices and interior volume of thebase material. Excess solution from the surface of the membrane may beremoved. The treated membrane is then immediately placed into an oven todry. Oven temperatures may range from 60°-200° C., but preferably120°-160° C. Drying the treated membrane in the oven causes the ionexchange resin to become securely adhered to both the external andinternal membrane surfaces, i.e., the fibrils and/or nodes of the basematerial. Additional solution application steps, and subsequent drying,may be repeated until the membrane becomes completely transparent.Typically, between 2 to 8 treatments are required, but the actual numberof treatments is dependent on the surfactant concentration and thicknessof the membrane. If the membrane is prepared without a supportstructure, both sides of the membrane may be treated simultaneouslythereby reducing the number of treatments required.

The oven treated membrane is then soaked in a solvent, such as the typedescribed hereinabove, to remove the surfactant. Thereafter the membraneis boiled in a swelling agent and under a pressure ranging from about 1to about 20 atmospheres absolute thereby increasing the amount ofswelling agent the treated membrane is capable of holding.

Alternatively, the ion exchange material may be applied to the membranewithout the use of a surfactant. This procedure requires additionaltreatment with the ion exchange resin. However, this procedure does notrequire that the oven treated membrane be soaked in a solvent, therebyreducing the total number of process steps. A vacuum may also be used todraw the ion exchange material into the membrane. Treatment withoutsurfactant is made easier if the water content of the solution islowered. Partial solution dewatering is accomplished by slow partialevaporation of the ion exchange material solution at room temperaturefollowed by the addition of a non-aqueous solvent. Ideally, a fullydewatered solution can be used. This is accomplished in several steps.First, the ion exchange material is completely dried at roomtemperature. The resulting resin is ground to a find powder. Finally,this powder is redissolved in a solvent, preferably a combination ofmethanol and isopropanol.

Because the composite membrane of the present invention can be madethinner than a fabric or non-woven reinforced structure, it is possibleto transport ions at a faster rate than previously has been achieved,with only a slight lowering of the selectivity characteristics of themembrane.

The following testing procedures were employed on samples which wereprepared in accordance with the teachings of the present invention.

TEST PROCEDURES TENSILE TEST

Tensile tests were carried out on an Instron Model 1122 tensile strengthtester, in accordance with ASTM D 638-91. Machine parameters were set asfollows:

Cross head speed: 0.423 cm/sec.

Full Scale load range: 222.4N

Humidity (%): 50

Temperature: 22.8° C.

Grip Distance: 6.35 cm

Specimens were stamped out to conform with Type (II) of ASTM D638. Thespecimens had a width of 0.635 cm, and a gauge length of 2.54 cm.

THICKNESS

Thickness of the base material was determined with the use of a snapgauge (Johannes Kafer Co. Model No. F1000/302). Measurements were takenin at least four areas of each specimen. Thickness of the driedcomposite membrane was also obtained with the use of the snap gauge.Thicknesses of swollen samples were not measurable with the snap gaugedue to the compression or residual water on the surface of the swollenmembrane. Thickness measurements of the swollen membranes were also notable to be obtained with the use of scanning electron microscopy due tothe interferences with the swelling agents.

MOISTURE VAPOR TRANSMISSION RATE (MVTR)

A potassium acetate solution, having a paste like consistency, wasprepared from potassium acetate and distilled water. (Such a paste maybe obtained by combining 230 g potassium acetate with 100 g of water,for example.) This solution was placed into a 133 ml. polypropylene cup,having an inside diameter of 6.5 cm, at its mouth. An expandedpolytetrafluoroethylene (ePTFE) membrane was provided having a minimumMVTR of approximately 85,000 g/m²−24 hr. as tested by the methoddescribed in U.S. Pat. No. 4,862,730 to Crosby. The ePTFE was heatsealed to the lip of the cup to create a taut, leakproof, microporousbarrier containing the solution.

A similar ePTFE membrane was mounted to the surface of a water bath. Thewater bath assembly was controlled at 23° C.±plus or minus 0.2° C.,utilizing a temperature controlled room and a water circulating bath.

Prior to performing the MVTR test procedure, a sample to be tested wasallowed to condition at a temperature of 23° C. and a relative humidityof 50%. The sample to be tested was placed directly on the ePTFEmembrane mounted to the surface of the water bath and allowed toequilibrate for 15 minutes prior to the introduction of the cupassembly.

The cup assembly was weighed to the nearest 1/1000 g. and was placed inan inverted manner onto the center of the test sample.

Water transport was provided by a driving force defined by thedifference in relative humidity existing between the water in the waterbath and the saturated salt solution of the inverted cup assembly. Thesample was tested for 10 minutes and the cup assembly was then removedand weighed again within 1/1000 g.

The MVTR of the sample was calculated from the weight gain of the cupassembly and was expressed in grams of water per square meter of samplesurface area per 24 hours.

PEEL STRENGTH

Peel strength or membrane adhesion strength tests were conducted onmembrane samples prepared with reinforced backings. Test samples wereprepared having dimensions of 3 inches by 3.5 inches (7.62 cm×8.89 cm).Double coated vinyl tape (type—#419 available from the 3M Company ofSaint Paul, Minn.) having a width of 1 inch (2.54 cm) was placed overthe edges of a 4 inch by 4 inch (10.2 cm×10.2 cm.) chrome steel plate sothat tape covered all edges of the plate. The membrane sample was thenmounted on top of the adhesive exposed side of the tape and pressure wasapplied so that sample was adhesively secured to the chrome plate.

The plate and sample were then installed, in a horizontal position,within an Instron tensile test machine Model No. 1000. An uppercrosshead of the tensile test machine was lowered so that the jaws ofthe test machine closed flat and tightly upon the sample. The uppercrosshead was then slowly raised pulling the membrane sample from thereinforced backing. When the membrane detached from the reinforcedbacking, the test was complete. Adhesion strength was estimated from theaverage strength needed to pull the membrane from the reinforcedbacking.

IONIC CONDUCTANCE

The ionic conductance of the membrane was tested using a Palico 9100-2type test system. This test system consisted of a bath of 1 molarsulfuric acid maintained at a constant temperature of 25° C. Submergedin the bath were four probes used for imposing current and measuringvoltage by a standard “Kelvin” four-terminal measurement technique. Adevice capable of holding a separator, such as the sample membrane to betested, was located between the probes. First, a square wave currentsignal was introduced into the bath, without a separator in place, andthe resulting square wave voltage was measured. This provided anindication of the resistance of the acid bath. The sample membrane wasthen placed in the membrane-holding device, and a second square wavecurrent signal was introduced into the bath. The resulting square wavevoltage was measured between the probes. This was a measurement of theresistance due to the membrane and the bath. By subtracting this numberfrom the first, the resistance due to the membrane alone was found.

DIMENSIONAL STABILITY

Reverse expansion in the x and y direction upon dehydration was measuredusing a type Thermomechanical Analyzer 2940, made by TA Instruments,Inc., of New Castle, Del. This instrument was used to apply apredetermined force to a sample that had been boiled in water for 30minutes. A quartz probe placed in contact with the sample measured anylinear changes in the sample as it dried. A sample was placed in aholder and then dried at 75° C. for greater than 10 min. The change indimension (i.e., the shrinkage) was recorded as a percentage of theoriginal weight.

WEIGHT LOSS WITH TEMPERATURE

A high resolution TGA 2950, Thermogravimetric Analyzer, made by TAInstruments (Newcastle, Del.) was used to determine the weight loss ofsamples with respect to temperature. This weight loss is an indicationof the water content of the ionomer sample.

SELECTIVITY

Two solutions of KCl, having concentrations of 1 molar and 0.5 molar,respectively, were separated using the membranes of the presentinvention. Two calomel reference electrodes (available from FischerScientific, Pittsburgh Pa., catalog number 13-620-52) were placed ineach solution, and the potential difference across the membranes wasrecorded using a digital multimeter (available from Hewlett Packard,Englewood Calif., catalog number HP34401A). The values obtainedcorrespond to the difference of chloride ion activity across themembrane and are reduced by the rate of anion migration across themembranes. Therefore the obtained values provide an indication of themembrane selectivity. The higher the measured voltage, the better themembrane selectivity.

BUBBLE POINT TEST

Liquids with surface free energies less than that of stretched porousPTFE can be forced out of the structure with the application of adifferential pressure. This clearing will occur from the largestpassageways first. A passageway is then created through which bulk airflow can take place. The air flow appears as a steady stream of smallbubbles through the liquid layer on top of the sample. The pressure atwhich the first bulk air flow takes place is called the bubble point andis dependent on the surface tension of the test fluid and the size ofthe largest opening. The bubble point can be used as a relative measureof the structure of a membrane and is often correlated with some othertype of performance criteria, such as filtration efficiency.

The Bubble Point was measured according to the procedures of ASTMF316-86. Isopropyl alcohol was used as the wetting fluid to fill thepores of the test specimen.

The Bubble Point is the pressure of air required to displace theisopropyl alcohol from the largest pores of the test specimen and createthe first continuous stream of bubbles detectable by their rise througha layer of isopropyl alcohol covering the porous media. This measurementprovides an estimation of maximum pore size.

PORE SIZE AND PORE SIZE DISTRIBUTION

Pore size measurements are made by the Coulter Porometer™, manufacturedby Coulter Electronics, Inc., Hialeah, Fla. The Coulter Porometer is aninstrument that provides automated measurement of pore sizedistributions in porous media using the liquid displacement method(described in ASTM Standard E1298-89). The Porometer determines the poresize distribution of a sample by increasing air pressure on the sampleand measuring the resulting flow. This distribution is a measure of thedegree of uniformity of the membrane (i.e., a narrow distribution meansthere is little difference between the smallest and largest pore size).The Porometer also calculates the mean flow pore size. By definition,half of the fluid flow through the filter occurs through pores that areabove or below this size. It is the mean flow pore size which is mostoften linked to other filter properties, such as retention ofparticulates in a liquid stream. The maximum pore size is often linkedto the Bubble Point because bulk air flow is first seen through thelargest pore.

BALL BURST TEST

This text measures the relative strength of a sample by determining themaximum load at break. The sample is challenged with a 1 inch diameterball while being clamped between two plates. The material is placed tautin the measuring device and pressure applied with the ball burst probe.Pressure at break is recorded.

AIR FLOW DATA

The Gurley air flow test measures the time in seconds for 100 cc of airto flow through a one square inch sample at 4.88 inches of waterpressure. The sample is measured in a Gurley Densometer (ASTM 0726-58).The sample is placed between the clamp plates. The cylinder is thendropped gently. The automatic timer (or stopwatch) is used to record thetime (seconds) required for a specific volume recited above to bedisplaced by the cylinder. This time is the Gurley number.

The Frazier air flow test is similar but is mostly used for much tinneror open membranes. The test reports flow in cubic feet per minute persquare foot of material at 0.5 inches water pressure. Air flow can alsobe measured with the Coulter Porometer. In this test, the operator canselect any pressure over a wide range. The Porometer can also perform apressure hold test that measures air flow during a decreasing pressurecurve.

BACKGROUND OF EXAMPLES

As may be appreciated by one skilled in the art, the present inventionprovides for an integral composite membrane. No porous surfaces areexposed in the present invention.

The integral composite membrane of the present invention can beadvantageously employed in electrolytic processes and chemicalseparations. In a plate-and-frame type electrodialysis unit, themembrane of the present invention would take the place of existingcation exchange membranes. This membrane could be of the type which islaminated to a spacer screen in accordance with a specific application.Due to the higher conductance of this membrane feasible with thinnermembranes, an electrodialysis unit could employ less membrane to achievea given flux rate, thereby saving space and cost. If equipment isretrofitted with this membrane, the voltage requirements would bereduced at a given current, or higher current could be run at a givenvoltage. Also, in a diffusion dialysis system, a given unit employingthe membrane of the present invention would provide a higher flux.

A fuel cell, utilizing the membrane of the present invention, operatesat a higher voltage for a given current density due to the improvedionic conductance of thinner versions of the membrane of this invention.

Due to improved water transport across the membrane of the presentinvention, high limiting current may be achieved with less fuel gashumidification, as compared to membranes which have been employedheretofore. For example, the membrane of the present invention has aresistance of 0.044 ohm-sq cm. At a current density of 1 A/cm², thiscauses a voltage drop of about 44 mV, or about a 99 mV improvement incell voltage compared to NAFION 117 membranes which have a resistance of0.143 Ω-cm³. (NAFION is a registered trademark of E. I. DuPont deNemours and Co., Inc.). As used herein, NAFION 117 means a membranehaving a thickness of 7 mils made from perfluorosulfonicacid/tetrafluoroethylene (TFE)/copolymer. This may reduce losses byabout 99 mW/sq cm at this operating condition for resistance. If thecell operating voltage increased from 500 mV to 599 mV, the cell voltageefficiency would increase from 41% to 49% of the theoretical 1.23 V. Thedecrease in the internal resistance of the cell allows the design ofsmaller or more efficient cells.

Without intending to limit the scope of the present invention, theapparatus and method of production of the present invention may bebetter understood by referring to the following examples. All samples ofePTFE provided in the following examples were made in accordance withthe teachings of U.S. Pat. No. 3,593,566. More particularly, the ePTFEhad the following material properties:

TYPE 1 TYPE 2 Gurley (sec.) 3.3 0.9 Bubble Point (psi) 28.3 32.6Mass/Area (g/m²) 6.1 4.4 Density (g/cc) 0.65 0.77 Longitudinal MaximumLoad (lbs.) 1.76 2.18 Transverse Maximum Load (lbs.) 2.33 1.31

As may be appreciated by one skilled in the art, ePTFE membranes can bemade with a wide range of physical property values, with ranges farexceeding the two examples given above.

EXAMPLE 1

A TYPE 1 ePTFE membrane, having a nominal thickness of 0.75 mils (0.02mm), was mounted on a 6 inch diameter wooden embroidery hoop. An ionexchange material/surfactant solution was prepared comprising 95% byvolume of a perfluorosulfonic acid/tetrafluoroethylene copolymer resinsolution (in H+ form, which itself is comprised of 5% perfluorosulfonicacid/tetrafluoroethylene copolymer resin, 45% water, and 50% a mixtureof low molecular weight alcohols, commercially available from E. I.DuPont de Nemours, Inc. under the registered trademark NAFION® typeNR-50 (1100 EW) hereinafter “NR-50”) and a 5% of a nonionic surfactantof octylphenoxy polyethoxyethanol (Triton X-100, commercially availablefrom Rohm & Haas of Philadelphia, Pa.). This solution was brushed onboth sides of the membrane to impregnate and substantially occlude theinterior volume of the membrane. The sample was then dried in the ovenat 140° C. for 30 seconds. The procedure was repeated two more times tofully occlude the interior volume. The sample was then soaked inisopropanol for 5 minutes to remove the surfactant. After rinsing withdistilled water and allowing the sample to dry at room temperature, afinal coat of the ion exchange material/surfactant solution was applied.The wet membrane was again dried in the oven at 140° C. for 30 secondsand soaked in isopropanol for 2 minutes. The membrane was finally boiledin distilled water for 30 minutes under atmospheric pressure to swellthe treated membrane. Gurley numbers for this material are summarized inTable 3. Ionic conductive rates are summarized in Table 4. The tensilestrength may be found in Table 2. Percent weight change of this samplemay be found in Table 6. The swollen membrane was later dried to adehydrated state in an oven at 140° C. for 30 seconds. The thickness ofthe dried composite membrane was measured and found to be approximatelythe same thickness as the base material.

EXAMPLE 2

A TYPE 1 ePTFE membrane, having a nominal thickness of 0.75 mils (0.02mm) and a Gurley Densometer air flow of 2-4 seconds, was placed on topof a netting of polypropylene obtained from Conwed Plastics Corp. ofMinneapolis, Minn. The two materials were bonded together on a laminatorwith 10 psig pressure, a speed of 15 feet per minute and a temperatureof 200° C. No adhesives were used. The reinforced membrane sample wasthen placed on a 6 inch wooden embroidery hoop. A solution was preparedof 96% by volume of a perfluorosulfonic acid/TFE copolymer resin inalcohol, and 4% of the nonionic surfactant Triton X-100. This solutionwas brushed only on the membrane side to substantially occlude theinterior volume of the membrane. The sample was dried in an oven at 130°C. This procedure was repeated three more times to fully occlude theinterior volume of the membrane. The sample was then baked in an oven at140° C. for 5 minutes. The sample was soaked in isopropanol for 5minutes to remove the surfactant. The membrane was then boiled indistilled water for 30 minutes under atmospheric pressure causing thetreated membrane to swell. Gurley numbers for this material aresummarized in Table 3.

This sample was tested for its peel strength in accordance with themethod described above. The linear bond strength was found to be 2.06lb./sq. in. (1450 kg/m²).

EXAMPLE 3

A TYPE 2 ePTFE membrane, having a thickness of 0.5 mils (0.01 mm), wasmounted on a 6 inch diameter wooden embroidery hoop. A solution of 100%by volume of NR-50 was brushed onto both sides of the membrane, withoutthe addition of any surfactants, to substantially occlude the interiorvolume of the membrane. The sample was then placed in an oven at 140° C.to dry. This procedure was repeated four more times until the membranewas completely transparent and the interior volume of the membrane wasfully occluded. The sample was then boiled in distilled water for 30minutes at atmospheric pressure causing the membrane to swell. Gurleynumbers for this material are summarized in Table 3.

EXAMPLE 4

A TYPE 2 ePTFE membrane, having a thickness of 0.5 mils (0.01 mm), wasmounted onto a 6 inch diameter wooden embroidery hoop. A solution wasprepared of 95% by volume NE-50 and 5% of the nonionic surfactant,Triton X-100. The solution was brushed on both sides of the membranewith a foam brush and the excess was wiped off. The wet membrane wasdried in an oven at 140° C. for 30 seconds. Three additional coats ofsolution were applied to the membrane in the same manner to fullyocclude the interior volume of the membrane. The membrane was thensoaked in isopropanol for 2 minutes to remove the surfactant. Themembrane was rinsed with distilled water and allowed to dry at roomtemperature. A final treatment of the solution was applied. The wetmembrane was dried in the oven at 140° C. for 30 seconds, and thensoaked in isopropanol for 2 minutes. Finally, the membrane was boiled indistilled water for 5 minutes. Moisture vapor transmission rates for thetreated membrane were measured and are summarized in Table 1.

EXAMPLE 5

A TYPE 1 ePTFE membrane, having a nominal thickness of 0.75 mils (0.02mm), was mounted onto a 6 inch diameter wooden embroidery hoop. TheGurley Densometer air flow for this membrane was 2-4 seconds. A solutionwas prepared of 95% by volume NR-50 and 5% Triton X-100. The solutionwas brushed on both sides of the membrane with a foam brush and theexcess was wiped off. The wet membrane was dried in the oven at 140° C.for 30 seconds. Three additional coats of solution were applied in thesame manner. The membrane was then soaked in isopropanol for 2 minutes.After rinsing with distilled water and allowing to dry at roomtemperature, a final coat of the solution was applied. The wet membranewas dried in the oven at 140° C. for 30 seconds, then soaked inisopropanol for 2 minutes. This material was not boiled. No swellingother than the minor swelling during the surfactant removal occurred.The ionic conduction rate for this material is presented in Table 4.

EXAMPLE 6

A TYPE 1 ePTFE membrane, having a nominal thickness of 0.75 mils (0.02mm), was mounted onto a 5 inch diameter plastic embroidery hoop. TheGurley Densometer air flow for this membrane was 2-4 seconds. A solutionwas prepared of 95% NR-50 and 5% Triton X-100. The solution was brushedon both sides of the membrane with a foam brush and the excess was wipedoff. The wet membrane was dried in the oven at 140° C. for 30 seconds.Two additional coats of solution were applied in the same manner tofully occlude the interior volume of the membrane. The membrane was thensoaked in isopropanol for 2 minutes. After rinsing with distilled waterand allowing to dry at room temperature, a final coat of the solutionwas applied. The wet membrane was dried in the oven at 140° C. for 30seconds, and then soaked in isopropanol for 2 minutes to remove thesurfactant. The sample was rinsed and dried at room temperature.

This sample was weighed before it was mounted on the 5 inch plastichoop. Following treatment, it was removed from the hoop and weighedagain. The ion exchange polymer content was directly calculated bydetermining the weight change before and after treatment. The ionexchange content for this sample was found to be 98.4 mg or 7.77 gramsper square meter of membrane.

EXAMPLE 7

A TYPE 1 ePTFE membrane, having a nominal thickness of 0.75 mils (0.02mm) and a Gurley Densometer air flow of 2-4 seconds, was placed on topof a netting of polypropylene which was obtained from Applied ExtrusionTechnologies, Inc. of Middletown, Del. The two materials were bondedtogether on a laminator with 10 psig pressure, a speed of 15 feet perminute and a temperature of 200° C. The reinforced sample was thenmounted on a 6 inch diameter wooden embroidery hoop. A solution wasprepared consisting of the following: 95% by volume NR-50, containing 5%by weight perfluorosulfonic acid/TFE copolymer resin in a solventmixture of less than 25% water, preferably 16-18% water, and theremainder a mixture of isopropanol and normal propanol: and 5% of TritonX-100 non-ionic surfactant. The solution was brushed on both sides ofthe membrane with a foam brush and the excess was wiped off. The wetmembrane was dried in an oven at 140° C. for 30 seconds. Threeadditional coats of solution were applied to the membrane in the samemanner to fully occlude the interior volume of the membrane. Themembrane was then soaked in isopropanol for 2 minutes to remove thesurfactant. The membrane was rinsed with distilled water and allowed todry at room temperature. A final treatment of the ion exchangematerial/surfactant solution was applied. The wet membrane was dried inthe oven at 140° C. for 30 seconds, then soaked in isopropanol for 2minutes. Finally, the membrane was boiled in distilled water for 5minutes.

EXAMPLE 8

A TYPE 1 ePTFE membrane, having a nominal thickness of 0.75 mils (0.02mm) and a Gurley Densometer air flow of 2-4 seconds, was mounted on a 6inch diameter wooden embroidery hoop. A solution was prepared consistingof the following: 95% NR-50, containing 5% by weight perfluorosulfonicacid/TFE copolymer resin in a solvent mixture of less than 25% water,preferably 16-18% water, and the remainder a mixture being isopropanoland normal propanol; and 5% of Triton X-100 non-ionic surfactant. Thesolution was brushed on both sides of the membrane with a foam brush andthe excess was wiped off. The wet membrane was dried in an oven at 140°C. for 30 seconds. Three additional coats of solution were applied tothe membrane in the same manner. The membrane was then soaked inisopropanol for 2 minutes to remove the surfactant. The membrane wasrinsed with distilled water and allowed to dry at room temperature. Afinal treatment of the solution was applied. The wet membrane was driedin the oven at 140° C. for 30 seconds, then soaked in isopropanol for 2minutes. Finally, the membrane was boiled in distilled water for 5minutes.

EXAMPLE 9

A TYPE 1 ePTFE membrane, having a nominal thickness of 0.75 mils (0.02mm) and a Gurley Densometer air flow of 2-4 seconds, was mounted on a 6inch diameter wooden embroidery hoop. The membrane was first submergedin a solution consisting of 25% Triton X-100 non-ionic surfactant, 25%water, and 50% isopropyl alcohol. Next, a solution of NR-50 was brushedon both sides of the membrane with a foam brush and the excess was wipedoff. The wet membrane was dried in an oven at 140° C. for 30 seconds.Three additional coats of surfactant solution followed by a coat ofNR-50 solution were applied to the membrane in the same manner to fullyocclude the interior volume of the membrane. The membrane was thensoaked in isopropanol for 2 minutes to remove the surfactant. Themembrane was rinsed with distilled water and allowed to dry at roomtemperature. A final treatment of the ion exchange material/surfactantwas applied to the membrane. The wet membrane was dried in the oven at140° C. for 30 seconds, then soaked in isopropanol for 2 minutes.Finally, the membrane was boiled in distilled water for 5 minutes.

EXAMPLE 10

A TYPE 1 ePTFE membrane, having a nominal thickness of 0.75 mils (0.02mm) and a Gurley Densometer air flow of 2-4 seconds, was mounted on a 6inch diameter wooden embroidery hoop. The membrane was first submergedin a solution consisting of 25% Triton X-100 non-ionic surfactant, 25%water, and 50% isopropyl alcohol. Next, a 95% by weight NR-50 solutioncontaining 5% by weight perfluorosulfonic acid/TFE copolymer resin in asolvent mixture of less than 25% water, preferably 16-18% water, and theremainder a mixture of isopropanol and normal propanol, was brushed onboth sides of the membrane with a foam brush and the excess was wipedoff. The wet membrane was dried in an oven at 140° C. for 30 seconds.Three additional coats of surfactant solution followed by the NR-50solution were applied to the membrane in the same manner to fullyocclude the interior volume of the membrane. The membrane was thensoaked in isopropanol for 2 minutes to remove the surfactant. Themembrane was rinsed with distilled water and allowed to dry at roomtemperature. A final treatment of the NR-50 solution was applied. Thewet membrane was fried in the oven at 140° C. for 30 seconds, thensoaked in isopropanol for 2 minutes. Finally, the membrane was boiled indistilled water for 5 minutes.

EXAMPLE 11

A TYPE 1 ePTFE membrane, having a nominal thickness of 0.75 mils (0.02mm) and a Gurley Densometer air flow of 2-4 seconds, was placed on topof a netting of polypropylene. The two materials were bonded together ona laminator with 10 psig pressure, a speed of 15 feet per minute and atemperature of 200° C. The reinforced sample was then mounted on a 6inch diameter wooden embroidery hoop. The membrane was first submergedin a solution consisting of 25% Triton X-100 non-ionic surfactant, 25%water, and 50 % isopropyl alcohol. Next, a solution of NR-50 was brushedon both sides of the membrane with a foam brush and the excess was wipedoff. The wet membrane was dried in an oven at 140° C. for 30 seconds.Three additional coats of the surfactant solution followed by the NR-50solution were applied to the membrane in the same manner to fullyocclude the interior volume of the membrane. The membrane was thensoaked in isopropanol for 2 minutes to remove the surfactant. Themembrane was rinsed with distilled water and allowed to dry at roomtemperature. A final treatment of the NR-50 solution was applied. Thewet membrane was dried in the oven at 140° C. for 30 seconds, thensoaked in isopropanol for 2 minutes. Finally, the membrane was boiled indistilled water for 5 minutes.

EXAMPLE 12

A TYPE 1 ePTFE membrane, having a nominal thickness of 0.75 mils (0.02mm) and a Gurley Densometer air flow of 2-4 seconds, was mounted on a 6inch diameter wooden embroidery hoop. A solution consisting 5% by weightof perfluorosulfonic acid/TFE copolymer resin in a solvent mixture ofless than 25% water, preferably 16-18% water, and the remainder amixture of isopropanol and normal propanol was allowed to evaporateslowly at room temperature. The resulting resin was ground to a powderwith a mortar and pestle. This resin was then dissolved in methanolunder low heat (less than 70° C.). The final solution contained theoriginal resin content in a base solvent of methanol such that the resincontent of the solution was 5% by weight. The solution was brushed onboth sides of the membrane with a foam brush and the excess was wipedoff. The wet membrane was dried in an oven at 140° C. for 30 seconds.Three additional coats of solution were applied to the membrane in thesame manner to fully occlude the interior volume of the membrane. Themembrane was boiled in distilled water for 5 minutes.

EXAMPLE 13

A TYPE 1 ePTFE membrane, having a nominal thickness of 0.75 mils (0.02mm) and a Gurley Densometer air flow of 2-4 seconds, was mounted on a 6inch diameter wooden embroidery hoop. A solution consisting of 5% byweight of perfluorosulfonic acid/TFE copolymer resin, in a solventmixture of less than 25% water, preferably 16-18% water, and theremainder a mixture of isopropanol and normal propanol, was allowed toevaporate slowly at room temperature. The resulting resin was ground toa powder with a mortar and pestle. This resin was then dissolved inmethanol under low heat (less than 70° C.). The final solution containedthe original resin content in a base solvent of methanol such that theresin content of the solution was 5% by weight. This solution was usedto prepare a new solution comprised of a 95% dewatered resin solution,and 5% Triton X-100 non-ionic surfactant. The solution was brushed onboth sides of the membrane with a foam brush and the excess was wipedoff. The wet membrane was dried in an oven at 140° C. for 30 seconds.Two additional coats of solution were applied to the membrane in thesame manner to fully occlude the interior volume of the membrane. Themembrane was then soaked in isopropanol for 2 minutes to remove thesurfactant. The membrane was rinsed with distilled water and allowed todry at room temperature. A final treatment of the resin/Triton X-100non-ionic surfactant solution was applied. The wet membrane was dried inthe oven at 140° C. for 30 seconds, then soaked in isopropanol for 2minutes. Finally, the membrane was boiled in distilled water for 5minutes.

EXAMPLE 14

A TYPE 1 ePTFE membrane, having a nominal thickness of 0.75 mils (0.02mm) and a Gurley Densometer air flow of 2-4 seconds, was mounted on a 6inch diameter wooden embroidery hoop. A solution consisting of 5% byweight of perfluorosulfonic acid/TFE copolymer resin in a solventmixture of less than 25% water, preferably 16-18% water, and theremainder a mixture of isopropanol and normal propanol, was allowed topartially evaporate slowly at room temperature. Before all the solventevaporated, the viscous liquid was mixed with methanol. The watercontent of the resulting solution was estimated at 5%. The resin contentof the solution was 5%. The solution was brushed on both sides of themembrane with a foam brush and the excess was wiped off. The wetmembrane was dried in an oven at 140° C. for 30 seconds. Threeadditional coats of solution were applied to the membrane in the samemanner to fully occlude the interior volume of the membrane. Themembrane was boiled in distilled water for 5 minutes.

EXAMPLE 15

A TYPE 1 ePTFE membrane, having a nominal thickness of 0.75 mils (0.02mm) and a Gurley Densometer air flow of 2-4 seconds, was placed on topof a netting of polypropylene. The two materials were bonded together ona laminator with 10 psig pressure, a speed of 15 feet per minute and atemperature of 200° C. The reinforced sample was then mounted on a 6inch diameter wooden embroidery hoop. A solution consisting of 5% byweight of perfluorosulfonic acid/TFE copolymer resin in a solventmixture of less than 25% water, preferably 16-18% water and theremainder a mixture of isopropanol and normal propanol, was allowed topartially evaporate slowly at room temperature. Before all the solventevaporated, the viscous liquid was mixed with methanol. The watercontent of the resulting solution was estimated at 5%. The resin contentof the solution was 5%. The solution was brushed on both sides of themembrane with a foam brush and the excess was wiped off. The wetmembrane was dried in an oven at 140° C. for 30 seconds. Threeadditional coats of solution were applied to the membrane in the samemanner to fully occlude the interior volume of the membrane. Themembrane was boiled in distilled water for 5 minutes.

EXAMPLE 16

A TYPE 1 ePTFE membrane, having a nominal thickness of 0.75 mils (0.02mm) and a Gurley Densometer air flow of 2-4 seconds, was mounted on a 6inch diameter wooden embroidery hoop. A solution consisting of 5% byweight of perfluorosulfonic acid/TFE copolymer resin in a solventmixture of less than 25% water, preferably 16-18% water and theremainder a mixture of isopropanol and normal propanol, was allowed topartially evaporate slowly at room temperature. Before all the solventevaporated, the viscous liquid was mixed with methanol. The watercontent of the resulting solution was estimated at 5%. The resin contentof the solution was 5%. This solution was used to prepare a new solutioncomprised of 95% of the low-water resin solution, and 5% of the nonionicsurfactant, Triton X-100. The new solution was brushed on both sides ofthe membrane with a foam brush and the excess was wiped off. The wetmembrane was dried in an oven at 140° C. for 30 seconds. Two additionalcoats of solution were applied to the membrane in the same manner tofully occlude the interior volume of the membrane. The membrane was thensoaked in isopropanol for 2 minutes to remove the surfactant. Themembrane was rinsed with distilled water and allowed to dry at roomtemperature. A final treatment of the new solution was applied. The wetmembrane was dried in the oven at 140° C. for 30 seconds, then soaked inisopropanol for 2 minutes. Finally, the membrane was boiled in distilledwater for 5 minutes.

EXAMPLE 17

A thermoplastic frame was cut and a membrane of ePTFE was placed at acenter location of the frame. The ePTFE membrane was heat sealed to theframe. The membrane was then treated in accordance with Example 1.Alternatively, a fluoroionomer membrane made in accordance with Example1 was secured mechanically within a frame.

This “framed” fluoroionomer composite has utility, by providing aunitary construction which can be placed in a device, which beyondserving as an ion exchange medium, can also serve as a sealant betweenvarious components of a cell assembly.

EXAMPLE 18

TEFLON® fine powder was blended with ISOPAR K mineral spirit at a rateof 115 cc per pound of fine powder. The lubricated powder was compressedinto a cylinder and was ram extruded at 70° C. to provide a tape. Thetape was split into two rolls, layered together and compressed betweenrolls to a thickness of 0.030 inch. Next, the tape was stretchedtransversely to 2.6 times its original width. The ISOPAR K was drivenoff by heating to 210° C. The dry tape was expanded longitudinallybetween banks of rolls in a heat zone heated to 300° C. The ratio ofspeed of the second bank of rolls to the first bank of rolls was 35:1and the third bank of rolls to the second bank of rolls was 1.5:1, for atotal of 52:1 longitudinal expansion producing a tape having a width of3.5 inches. This tape was heated to 295° C. and transversely expanded13.7 times in width, while being constrained from shrinkage and thenheated at 365° C. while still constrained. This process produced aweb-like membrane having a porous microstructure composed substantiallyof fibrils in which no nodes were present.

EXAMPLE 19

An ePTFE membrane, having a nominal thickness of 2.2 mils (0.6 mm) and aGurley Densometer air flow of 6-9 seconds, was mounted on a 6 inchdiameter wooden embroidery hoop. A solution consisting of 5% by weightof ionomer, such as perfluorosulfonic acid/TFE copolymer resin in asolvent such as methanol, was brushed on both sides of the membrane witha foam brush and the excess was wiped off. The wet membrane was dried inan oven at 140° C. for 30 seconds. Three additional coats of solutionwere applied to the membrane in the same manner to fully occlude theinterior volume of the membrane.

EXAMPLE 20

An ePTFE membrane, having a nominal thickness of 3 mils (0.8 mm) and aGurley Densometer air flow of 6-9 seconds, was mounted on a 6 inchdiameter wooden embroidery hoop. A solution consisting of 5% by weightof ionomer, such as perfluorosulfonic acid/TFE copolymer resin in asolution such as methanol, was brushed on both sides of the membranewith a foam brush and the excess was wiped off. The wet membrane wasdried in an oven at 140° C. for 30 seconds. Three additional coats ofsolution were applied to the membrane in the same manner to fullyocclude the interior volume of the membrane.

EXAMPLE 21

An ePTFE membrane, having a nominal thickness of 0.75 mils (0.02 mm) anda Gurley Densometer air flow of 2-4 seconds, was mounted on a 6 inchdiameter wooden embroidery hoop. A solution consisting of 5% by weightof ionomer, such as perfluorosulfonic acid/TFE copolymer resin of 1100EW in a solvent such as methanol, was brushed on both sides of themembrane with a foam brush and the excess was wiped off. The wetmembrane was dried in an oven at 140° C. for 30 seconds. Threeadditional coats of solution were applied to the membrane in the samemanner to fully occlude the interior volume of the membrane. A secondcomposite membrane prepared in the same manner, however using a 950 EWperfluorosulfonic acid/TFE copolymer in a solvent such as ethanol. Thetwo membranes were then combined (laminated) by use of heat andpressure. For example, at 190° C. (375° F.) @ 100 psi for 1 minute in aheated press or a comparable arrangement in a heated roll.

EXAMPLE 22

An ePTFE membrane, having a nominal thickness of 0.75 mils (0.002 mm)and a Gurley Densometer air flow of 2-4 seconds, was mounted on a 6 inchdiameter wooden embroidery hoop. An alcohol solution consisting of 5% byweight of ionomer, and a finely divided powder, such as carbon black(10%), was brushed on both sides of the membrane with a foam brush andthe excess was wiped off. The wet membrane was dried in an oven at 140°C. for 30 seconds. Three additional coats of solution were applied tothe membrane in the same manner to fully occlude the interior volume ofthe membrane. The final composite had a dark appearance.

EXAMPLE 23

An ePTFE membrane, having a nominal thickness of 0.75 mils (0.002 mm)and a Gurley Densometer air flow of 2-4 seconds, was mounted on a 6 inchdiameter wooden embroidery hoop. A solution consisting of 5% by weightof ionomer, was brushed on both sides of the membrane with a foam brushand the excess was wiped off. The wet membrane was dried in an oven at140° C. for 30 seconds. Three additional coats of solution were appliedto the membrane in the same manner to fully occlude the interior volumeof the membrane. This composite membrane was then combined (laminated)to another ePTFE membrane having a nominal thickness of 0.75 (0.002) mmand a Gurley Densometer air flow of 2-4 second, by use of heat andpressure (for example 190° C. [375° F.] @ 100 psi) using a heated pressor a comparable arrangement.

A solution consisting of 5% by weight of ionomer, such asperfluorosulfonic acid/TFE copolymer resin in a solvent such asmethanol, was brushed on the ePTFE membrane side of the membrane with afoam brush and the excess was wiped off. The wet membrane was dried inan oven at 140° C. for 30 seconds. Three additional coats of solutionwere applied to the membrane in the same manner to fully occlude theinterior volume of the ePTFE membrane. A thicker integral compositemembrane was thus formed.

Comparative Samples

NAFION 117, a perfluorosulfonic acid cation exchange membrane,unreinforced film of 1100 equivalent weight commercially available fromE. I. DuPont de Nemours Co., Inc., having a quoted nominal thickness of7 mils (0.18 mm) was obtained. The samples, originally in the hydratedswollen state, were measured in the x- and y-directions and weighed.

Without intending to limit the scope of the present invention, datacollected from testing the ion exchange membranes made in accordancewith the procedures of the foregoing examples are summarized in thefollowing tables. As may be appreciated by one skilled in the art, thesetables reveal that the ion exchange membrane of this invention hassuperior ionic conductance and exceptional dimensional stabilitycompared to known ion exchange membranes. Furthermore, this inventivemembrane has good mechanical strength in the unswollen state and retainsmuch of its mechanical strength in the swollen state, whereasconventional membranes are substantially weakened upon hydration.

TABLE 1 Moisture Vapor Transmission Rates (MVTR) Sample ID* MVTR(grams/m²-24 hrs.) 4 25.040 NAFION 117 23.608 *Measurements wereobtained on samples in their swollen state.

TABLE 2 Tensile Test (Avg) Normalized Stress @ Max Load (psi) Sample IDM-Dir XM-Dir Example 1 4706 2571 NAFION 117* 2308 1572 Example 6 49883463 NAFION 117*** 4314 3581 *sample was boiled in distilled water for30 minutes. ***sample was tested as received from E. I. DuPont deNemours, Inc.

TABLE 2 Tensile Test (Avg) Normalized Stress @ Max Load (psi) Sample IDM-Dir XM-Dir Example 1 4706 2571 NAFION 117* 2308 1572 Example 6 49883463 NAFION 117*** 4314 3581 *sample was boiled in distilled water for30 minutes. ***sample was tested as received from E. I. DuPont deNemours, Inc.

TABLE 2 Tensile Test (Avg) Normalized Stress @ Max Load (psi) Sample IDM-Dir XM-Dir Example 1 4706 2571 NAFION 117* 2308 1572 Example 6 49883463 NAFION 117*** 4314 3581 *sample was boiled in distilled water for30 minutes. ***sample was tested as received from E. I. DuPont deNemours, Inc.

TABLE 2 Tensile Test (Avg) Normalized Stress @ Max Load (psi) Sample IDM-Dir XM-Dir Example 1 4706 2571 NAFION 117* 2308 1572 Example 6 49883463 NAFION 117*** 4314 3581 *sample was boiled in distilled water for30 minutes. ***sample was tested as received from E. I. DuPont deNemours, Inc.

TABLE 2 Tensile Test (Avg) Normalized Stress @ Max Load (psi) Sample IDM-Dir XM-Dir Example 1 4706 2571 NAFION 117* 2308 1572 Example 6 49883463 NAFION 117*** 4314 3581 *sample was boiled in distilled water for30 minutes. ***sample was tested as received from E. I. DuPont deNemours, Inc.

TABLE 7 Transverse Direction Machine Direction Example 1  2.95%  2.90%NAFION 117 11.80% 10.55%

Although a few exemplary embodiments of the present invention have beendescribed in detail above, those skilled in the art readily appreciatethat many modifications are possible without materially departing fromthe novel teachings and advantages which are described herein.Accordingly, all such modifications are intended to be included withinthe scope of the present invention, as defined by the following claims.

Having described the invention, what is claimed is:
 1. A compositemembrane comprising n t least one expanded polytetrafluoroethylenemembrane having a porous microstructure of polymeric fibrils and havinga thickness of 80 microns or less; and (b) an t least one ion exchangematerial impregnated throughout the porous microstructure of theexpanded polytetraafluoroethylene membrane so as to render an interiorvolume of the expanded polytetrafluoroethylene membrane substantiallyocclusive, the impregnated expanded polytetrafluoroethylene membranehaving a Gurley number of greater than 10,000 seconds, wherein the ionexchange material substantially impregnates the membrane so as to renderan interior volume of the membrane substantially occlusive ; whereinoptionally the at least one ion exchange material is complimented bypowder, non-ionic polymer, or a combination thereof.
 2. The compositemembrane of claim 1, wherein the expanded polytetrafluoroethylenemembrane comprises consists essentially of a microstructure of nodesinterconnected by the fibrils.
 3. The composite membrane of claim 1,wherein the at least one ion exchange material is selected from a groupconsisting of: perfluorinated sulfonic acid resin, perfluorinatedcarboxylic acid resin, polyvinyl alcohol, divinyl benzene, styrene-basedpolymers, and metal salts.
 4. The composite membrane of claim 1, whereinthe at least one ion exchange material is comprised complimented atleast in part of a by powder.
 5. The composite membrane of claim 4,wherein the powder is at least in part carbon.
 6. The composite membraneof claim 4, wherein the powder is at least in part a metal.
 7. Thecomposite membrane of claim 4, wherein the powder is at least in part ametal oxide.
 8. The composite membrane of claim 1, wherein the ionexchange material is a perfluorosulfonic acid/tetrafluoroethylenecopolymer resin derived from a solvent solution selected from a groupconsisting essentially of water, ethanol, propanol, butanol, andmethanol.
 9. The composite membrane of claim 1, wherein the ion exchangematerial is at least in part a complimented by non-ionic polymer. 10.The composite membrane of claim 9, wherein the non-ionic polymer isthermoplastic resin.
 11. The composite membrane of claim 9, wherein thenon-ionic polymer is a thermoset resin.
 12. The composite membrane ofclaim 9, wherein the non-ionic polymer is polyolefin or fluoropolymer.13. The composite membrane of claim 4, wherein the powder is finelydivided.
 14. The composite membrane of claim 4, wherein the powder is anorganic powder.
 15. The composite membrane of claim 4, wherein thepowder is an inorganic powder.
 16. The composite membrane of claim 4,wherein the powder is selected from the group consisting of carbonblack, graphite, nickel, silica, titanium dioxide, and platinum black.17. The composite membrane of claim 4, wherein the powder is carbonblack.
 18. The composite membrane of claim 4, wherein the powder isgraphite.
 19. The composite membrane of claim 4, wherein the powder issilica.
 20. The composite membrane of claim 4, wherein the powder istitanium dioxide.
 21. The composite membrane of claim 4, wherein thepowder is platinum black.
 22. The composite membrane of claim 1, whereinthe thickness of the expanded polytetrafluoroethylene membrane is 60microns or less.
 23. The composite membrane of claim 1, wherein thethickness of the expanded polytetrafluoroethylene membrane is 40 micronsor less.
 24. The composite membrane of claim 1, wherein the thickness ofthe expanded polytetrafluoroethylene membrane is 20 microns or less. 25.The composite membrane of claim 1, wherein the thickness of the expandedpolytetrafluoroethylene membrane is at least 1.5 microns.
 26. Thecomposite membrane of claim 1, wherein the thickness of the expandedpolytetrafluoroethylene membrane is at least 13 microns.
 27. A compositemembrane according to claim 1, wherein the impregnated membrane has anionic conductance of at least 8.5 mhos/cm².
 28. A composite membraneaccording to claim 1, wherein the impregnated membrane has an ionicconductance of at least 22.7 mhos/cm².
 29. A composite membraneaccording to claim 1, wherein the impregnated membrane has been heatedto a temperature of 60° C. to 200° C.
 30. A composite membrane accordingto claim 29, wherein the impregnated membrane has an ionic conductanceof at least 8.5 mhos/cm².
 31. A composite membrane according to claim30, wherein the thickness of the expanded polytetrafluoroethylenemembrane is 20 microns or less and the ion exchange material isperfluorinated sulfonic acid resin.
 32. A composite membrane accordingto claim 31, wherein the impregnated membrane is prepared byimpregnation of at least two sides of the expandedpolytetrafluoroethylene membrane with the ion exchange material.
 33. Acomposite membrane according to claim 32, wherein the impregnation iscarried out by multiple impregnations of the at least two sides ofexpanded polytetrafluoroethylene membrane.
 34. A composite membraneaccording to claim 29, wherein the impregnated membrane has an ionicconductance of at least 22.7 mhos/cm².
 35. A composite membraneaccording to claim 1, wherein the impregnated membrane has been heatedto a temperature of 120° C. to 200° C.
 36. A composite membraneaccording to claim 1, wherein the impregnated membrane has been heatedto a temperature of 140° C. to 200° C.
 37. A laminate of compositemembranes consisting essentially of at least two composite membraneslaminated to each other, wherein the at least two composite membraneseach consist essentially of: (a) an expanded polytetrafluoroethylenemembrane having a porous microstructure of polymeric fibrils and havinga thickness of 80 microns or less; and (b) an ion exchange materialimpregnated throughout the porous microstructure of the membrane so asto render an interior volume of the expanded polytetrafluoroethylenemembrane substantially occlusive, the impregnated membrane having aGurley number of greater than 10,000 seconds.
 38. A laminate accordingto claim 37, wherein each of the impregnated polytetrafluoroethylenemembranes have been heated to a temperature of at least 60° C. and eachof the impregnated membranes have an ionic conductance of at least 22.7mhos/cm² ; wherein the thickness of each of the impregnated membranes is20 microns or less; wherein the ion exchange material is perfluorinatedsulfonic acid resin; wherein each of the impregnated membranes areprepared by multiple impregnations of two sides of the expandedpolytetrafluoroethylene membrane with ion exchange material.
 39. Alaminate according to claim 38, wherein the thickness of the laminate is40 microns or less.
 40. A laminate according to claim 38, wherein atleast two of the impregnated membranes are impregnated with ion exchangematerial before lamination to form the laminate.
 41. A laminateaccording to claim 37, wherein the thickness of the laminate is 40microns or less.
 42. A laminate according to claim 37, wherein at leasttwo of the impregnated membranes are impregnated with ion exchangematerial before lamination to form the laminate.
 43. A laminateaccording to claim 37, wherein the laminate is prepared by (i)impregnation of at least one first unimpregnated expandedpolytetrafluoroethylene membrane with ion exchange material to form afirst impregnated membrane, (ii) lamination of the first impregnatedmembrane with a second unimpregnated expanded polytetrafluoroethylenemembrane, and (iii) impregnation of the second unimpregnated expandedpolytetrafluoroethylene to form a second impregnated membrane which islaminated to the first impregnated membrane.
 44. A laminate according toclaim 37, wherein lamination is carried out with heat.
 45. A membranecomprising: (a) at least one expanded polytetrafluoroethylene membranehaving a porous microstructure of polymeric fibrils; and (b) at leastone ion exchange material impregnated throughout the porousmicrostructure of the membrane so as to render an interior volume of theexpanded polytetrafluoroethylene membrane substantially occlusive, theimpregnated expanded polytetrafluoroethylene membrane having a Gurleynumber of greater than 10,000 seconds, wherein powder is included withthe at least one ion exchange material.
 46. A membrane according toclaim 45, wherein the thickness of the expanded polytetrafluoroethylenemembrane is 80 microns or less.
 47. A membrane according to claim 45,wherein the thickness of the expanded polytetrafluoroethylene membraneis 20 microns or less.
 48. A membrane according to claim 47, wherein theimpregnated membrane has an ionic conductivity of at least 8.5 mhos/cm².49. A membrane according to claim 47, wherein the impregnated membranehas an ionic conductivity of at least 22.7 mhos/cm².
 50. A membraneaccording to claim 45, wherein the impregnated membrane has an ionicconductivity of at least 8.5 mhos/cm².
 51. A membrane according to claim45, wherein the impregnated membrane has an ionic conductivity of atleast 22.7 mhos/cm².
 52. A membrane consisting essentially of: (a) atleast one expanded base membrane having a porous microstructure ofpolymeric fibrils and having a thickness of 80 microns or less; and (b)at least one ion exchange material impregnated throughout the porousmicrostructure of the expanded base membrane so as to render an interiorvolume of the base membrane substantially occlusive the impregnatedexpanded base membrane having a Gurley number of greater than 10,000seconds.
 53. A membrane according to claim 52, wherein the impregnatedmembrane has a thickness of 20 microns or less and an ionic conductivityof at least 8.5 mhos/cm².
 54. A membrane according to claim 52, whereinthe impregnated membrane has an ionic conductivity of at least 22.7mhos/cm².
 55. A membrane according to claim 54, wherein the impregnatedmembrane is heated to between 120° C. and 200° C.
 56. A compositemembrane consisting essentially of: (a) at least one expandedpolytetrafluoroethylene membrane having a porous microstructure ofpolymeric fibrils and having a thickness of 80 microns or less; and (b)at least one ion exchange material impregnated throughout the porousmicrostructure of the expanded polytetrafluoroethylene membrane so as torender an interior volume of the expanded polytetrafluoroethylenemembrane substantially occlusive, the impregnated membrane not allowingfor fluid percolation.
 57. A composite membrane according to claim 56,wherein the ionic conductivity of the impregnated membrane is at least8.5 mhos/cm² and the impregnated membrane is heated to between 120° C.and 200° C.
 58. A composite membrane according to claim 56, wherein thethickness of the composite membrane and the thickness of the impregnatedexpanded polytetrafluoroethylene membrane are substantially the same.59. A composite membrane according to claim 56, wherein a side of thecomposite membrane is laminated to a support structure, and the ionexchange material is impregnated throughout the porous microstructure ofthe expanded polytetrafluoroethylene membrane from a side opposite toside laminated to the support structure.
 60. A composite membraneaccording to claim 56, wherein the ion exchange material is impregnatedthroughout the porous microstructure of the expandedpolytetrafluoroethylene membrane by simultaneous treatment of both sidesof the expanded polytetrafluoroethylene membrane.
 61. A compositemembrane according to claim 56, wherein the ion exchange material isuniformly impregnated throughout the porous microstructure of theexpanded polytetrafluoroethylene membrane, and the composite membranehas no porous surfaces exposed.